Transistors are often used as switches in electronic circuits, such as a USB switch. FIG. 1 shows a schematic diagram of a prior art electronic switching system 10. Electronic switching system 10 comprises a NMOS transistor switch N1, a switch S1, a current source I1, and an output capacitor CO. The transistor N1 has an input terminal IN, an output terminal OUT and a control terminal CTL, wherein the input terminal receives an input voltage VIN, and wherein the output terminal provides an output voltage VOUT. The switch S1 has a first terminal and a second terminal, wherein the first terminal is coupled to the control terminal CTL of the transistor N1, and wherein the second terminal is coupled to ground (GND). The first current source I1 has a first terminal and a second terminal, wherein the first terminal is coupled to a first power supply terminal SP1, and wherein the second terminal is coupled to the control terminal CTL of the transistor N1. The output capacitor CO has a first terminal and a second terminal, wherein the first terminal is coupled to the output terminal OUT of the transistor N1, and wherein the second terminal is coupled to ground GND. The first power supply terminal SP1 normally receives a first voltage VSP higher than the input voltage VIN to make the transistor N1 be completely turned on, such as VSP=VIN+5, or VSP=2×VIN that is provided by a 2 times charge pump. When the first switch S1 is turned on, the control terminal CTL of the transistor N1 is at a low potential, and the transistor N1 is turned off. When the first switch S1 is turned off, the first current source I1 pulls the control terminal CTL of the transistor N1 to the first voltage VSP, which makes the transistor N1 be completely turned on, and the output voltage VOUT is approximately equal to the input voltage VIN.
Since the USB protocol is a standard protocol, the input voltage VIN is relatively stable, around 5V. But, due to device failure or external power supply varying, the input voltage VIN and the output voltage VOUT may raise, which will be very easy to damage the external USB device coupled to be output terminal OUT of the transistor switch N1. So it is necessary to provide an over-voltage protection circuit for a transistor switch and USB interface related circuits.